1. Field of the Invention
The present invention relates to a mobile radio communication system of which a service area is composed of macro cells that are subdivided into micro cells.
2. Description of the Related Art
As a conventional mobile radio communication system, a related art reference disclosed in Japanese Patent Laid-Open Publication No. 5-130018 is known. In the mobile radio communication system, to solve problems of shortage of channels and reduction of occurrences of hand-off, as shown in FIG. 3, the service area is composed of macro cells 31, each of which is composed of micro cells 32. A terminal unit communicates with a macro cell radio base station or a micro cell radio base station corresponding to the traveling speed of the terminal unit.
In the mobile radio communication system, to allow a mobile station in the service area to communicate with a radio base station, a communication channel should be assigned for each communication session demanded for communication. Normally, the channel assignment is processed on the radio base station side (controlling station). Likewise, this process is performed in the above-described mobile radio communication system.
Thus, the radio base station side should perform many process steps for the management of communication channels such as checking out assignable communication channels, measuring the field intensity of a radio wave transmitted by each mobile station, determining the permission of a communication request issued by each mobile station, and monitoring a communication state (line quality) in the communication.
When a mobile station that is communicating with a radio base station moves to an adjacent radio area, to keep the communication, the original communication channel should be switched to an optimum channel that a radio base station in the adjacent radio area provides. In this case, in the conventional mobile radio communication system, as with selecting a channel in the communication start time, the radio base station side selects a channel for communicating with the radio base station in the adjacent radio area to which the mobile station will move.
To alleviate the process performed on the radio base station side, a method for causing each mobile station to select a communication channel has been used in some systems. However, in this method, since each mobile station only measures the field intensities of available channels and selects a channel with the strongest field intensity, the mobile station does not sufficiently have information about the system structure and the current system state. In addition, since each mobile station selects a channel corresponding to the field intensities of received channels, the process on the radio base station side is applied to the mobile station side. Thus, each mobile station occasionally fails to select an optimum channel.
As a first problem in such a conventional mobile radio communication system, when communications between mobile stations and a relevant radio base station get congested or when a new channel is assigned between each mobile station and the relevant radio base station or the current channel is switched to a new channel, the process performance on the radio base station side deteriorates.
This is because only the radio base station side performs a determining process and a selecting process for assigning a channel.
As a second problem in such a conventional mobile radio communication system, the radio base station side selects a channel necessary for communicating between the mobile station and the relevant radio base station. However, the selected channel is not optimum for the state of each mobile station. The communication quality of the selected channel may be so bad as the current channel should be switched to another channel just after the communication initiates. Alternatively, in this case, the communication may be immediately disconnected.
This is because the radio base station side does not have an effective means for monitoring the moving speed and so forth of each mobile station. For example, when a communication channel of a radio base station that has a very small radio area is assigned to a mobile station that is moving at high speed, the mobile station moves to an adjacent radio area in a short time. Thus, the communication quality rapidly deteriorates. In other words, an improper channel may be assigned to each mobile station.